Phthalocyanine compounds are useful as coatings, printing inks, catalysts, or electronic materials. In recent years, they have been extensively studied particularly for their use as electrophotographic photoreceptor materials, optical recording materials and photoelectric conversion materials.
In the field of electrophotographic photoreceptors, there has recently been an increasing demand to extend the photosensitive wavelength region of conventional organic photoconductive materials to a longer side of near infrared light (780 to 830 nm) which corresponds to a wavelength region of a semiconductor laser so as to make them applicable to a digital recording system, such as a laser printer. From this point of view, there have been reported photoconductive materials for semiconductor lasers, such as squarylium compounds as disclosed in JP-A-49-105536 and JP-A-58-21416, triphenylamine type tris-azo compounds as disclosed in JP-A-61-151659, and phthalocyanine compounds as disclosed in JP-A-48-34189 and JP-A-57-148745 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In cases where an organic photoconductive material is used as a photosensitive material for semiconductor lasers, they are required to have a photosensitive wavelength region extended to a longer side and to provide a photoreceptor having satisfactory sensitivity and durability. None of the above-described conventional organic photoconductive materials sufficiently satisfies these requirements.
In order to overcome the drawbacks of the conventional organic photoconductive materials, the relationship between their crystal form and electrophotographic characteristics has been studied. In particular, many reports have hitherto been made on phthalocyanine compounds.
It is known that phthalocyanine compounds generally exhibit several different crystal forms depending on the process of production or the process of treatment and that the difference in crystal form has a great influence on their photoelectric conversion characteristics. For example, known crystal forms of copper phthalocyanine compounds include .alpha.-, .epsilon.-, .pi.-, .chi.-, .rho.-, .gamma.-, and .delta.-forms as well as a stable .beta.-form. These crystal forms are known capable of interconversion by a mechanical strain, a sulfuric acid treatment, an organic solvent treatment, a heat treatment, and the like as described, e.g., in U.S. Pat. Nos. 2,770,629, 3,160,635, 3,708,292, and 3,357,989. Further, JP-A-50-38543 refers to the relationship between a crystal form of copper phthalocyanine and its electrophotographic characteristics.
JP-A-62-119547 discloses an electrophotographic photoreceptor using a dihalogenotin phthalocyanine compound as a charge generating material. JP-A-1-144057 discloses a tin phthalocyanine compound having specific peaks on its X-ray diffraction pattern and an electrophotographic photoreceptor using the same.
However, any of the known phthalocyanine compounds proposed to date is still unsatisfactory in photosensitivity and durability when used as a photosensitive material. Besides the performance problems, they need complicated manipulations for crystal transformation, or the crystal form is difficult to control.
Further, dichlorotin phthalocyanine compounds have poor dispersibility in a binder resin only to produce a dispersion having poor coating properties. As a result, photoreceptors using dichlorotin phthalocyanine compounds exhibit insufficient sensitivity characteristics and insufficient charge retention and also tend to cause image defects, such as fog and black spots, called black pepper.
Conventional tin phthalocyanine compounds have poor crystal form stability in a solvent. Therefore, when dispersed in a solvent or after being coated to form a photosensitive layer, the compound cannot maintain its crystal form for a sufficient period of time, failing to exhibit satisfactory performance properties as a charge generating material.
That is, if a dichlorotin phthalocyanine compound has too a small primary particle size, it has poor crystal stability in a solvent and is liable to be transformed to another crystal form. Conversely, if the primary particle size is too large, the resulting photoreceptor would suffer from marked reduction in sensitivity and stability. In addition, dichlorotin phthalocyanine compounds have another problem that the crystal form stability greatly depends on the kind of a dispersing solvent. The dichlorotin phthalocyanine crystal previously proposed by the present inventors cannot still get rid of the problem of poor crystal form stability in a solvent and is easily transformed to another crystal form, making it difficult to take full advantage of its electrophotographic characteristics.